[APWG] Invaders

[maryann whitman]

Here is a comment on the NYT article from another list. The last couple of
paragraphs are particularly relevant to this conversation.-Maryann

 

[Authors] point out that the invasion has not led to a mass extinction of
native plants. The number of documented extinctions of native New Zealand
plant species is a grand total of three

The key word here is documented - that is the plants that were discovered
and known about before they dissapeared.  The Department of Conservation's
NZ Threat Classification List lists six plants that are known to be extinct

Lepidium obtusatum Kirk
Brassicaceae      
Logania depressa Hook.f.
Loganiaceae 
Myosotis traversii var. cinerascens (Petrie) L.B.Moore      Boraginaceae
None
Known
Pseudognaphalium (a) (CHR 365358; Zoo)
Stellaria elatinoides Hook.f.
Caryophyllaceae   
Trilepidea adamsii (Cheeseman) Tiegham                      Loranthaceae
Adams
mistletoe

Descriptions for 5 of these species can be found on www.nzpcn.org.nz All but
the last are small herbs, that were found once or on a very small number of
occasions, indicating that they were probably not common.  Most were in very
restricted and specialised habitats.  The last is a reasonably large
mistletoe species and Historic records indicate that this species was never
common.

Many more species may have gone extinct without anybody noticing -
especially in unpleasant habitats (e.g. seabird roosts, swamps) or in very
small and unusual habitats.  

Furthermore extinction generally depends on the European observation of
plants.  It is not know how many plant species dissapeared when the Maori
(and Moriori) settled in New Zealand with their range of introduced species
(principally the Pacific rat or kiore Rattus exulans and dog, kuri Canis
familiaris)and their habit of burning vegetation.  Nor do we know how many
plant species dissapeared when pigs and goats were liberated by Europeans on
islands to act as food for potential cast aways.  Species may have
dissapeared before Europeans botanised an area, because Europeans were even
better at burning vegetation.

We suspect that invasive species are the cause of extinction for some
species (e.g. all large leaved New Zealand mistletoes seem to be favourite
food of the introduced Australian brush tail possum - Trichosurus vulpecula)
and that could certainly account for the dissapearnce of Trilepidea adamsii
on mainland NZ.  But Trilepidea adamsii was also found on Great Barrier
Island (NZ not Aus) and possums were never released there.

It is also difficult to definitifly say that invasive species caused the
demise of some of these plant species, unless you include humans as an
invasive species.  Some extinctions are more likely to have been caused by
habitat loss or modification.  Others may be caused indirectly through
invasive species, for instance guana loving plant species are highly
threatened because many mainland seabird colonies have been decimated by
introduced predators, the predators don't however eat these plant species as
far as we know.

So in a nutshell - I don't agree with the statement that there were only
three plant species known to have become extinct due to invasive species.
We don't know how many have gone extinct, and in many cases extinction is
more likely to have been through habitat destruction or modification by
humans rather than invasive species.

The case for extinction of animal species by invasive species is much
clearer, especially when you can find rodent or mustelid teeth marks on the
bones, or find piles of bones in human made ovens.

Astrid van Meeuwen-Dijkgraaf (PhD)